Kris, It is entirely possible the mother has created a perfect greenhouse
under those shells and has a fungal overgrowth on her nipples independent
from candida other places. In spite of the holes, my experiences with shells
have been that the environment inside them is pretty moist and airless, and
they promote almost constant leaking.  I think the pressure of the shells on
the breasts has contributed to some cases of plugged duct.  For sore nipples
I just recommend leaving bra flaps down or going bra-less and wearing cotton
t-shirts.  Tell her to do some vinegar and water soaks:  mix up a bowl of
warm water with white vinegar maybe 2 cups water: 1/2 cup vinegar and soak
for a few min.  Then air dry and treat area with monistat cream. Repeat the
monistat after every feeding for several days and the soaks 2-3 times a day.
See what happens in 3 days or so.  Sometimes candida can literally be washed
off if caught early before skin breaks down.
Barbara